Acoustooptic modulation element and system for acoustooptically carrying out modulation, of a plurality of parallel beams by the use of a single acoustooptic medium

ABSTRACT

Responsive to a plurality of parallel light beams and electric signals corresponding to the parallel light beams, an acoustic modulation element produces output light beams modulated by acoustic waves resulting from the electric signals. The element comprises a single acoustooptic medium block receiving the parallel beams on a first surface thereof and emitting the output light beams from a second surface opposite to the first surface. A plurality of transducer units are laid in parallel on a third surface of the block between the first and the second surfaces along the respective parallel beams so as to propagate the acoustic waves into the block when activated by the electric signals. The block may have a fourth surface opposite to the third surface and arcuately or triangularly recessed towards the third surface. Preferably, the transducer units are mounted on plateaus isolated from each other by a channel formed between two adjacent ones of the plateaus, in order to avoid interferences of the acoustic waves. The element is for use in combination with a beam splitter member for producing the parallel beams in response to a single light beam.

This is a division of application Ser. No. 517,346, filed July 26, 1983,now U.S. Pat. No. 4,592,621.

BACKGROUND OF THE INVENTION

This invention relates to an acoustooptic modulation element for use inacoustooptically modulating a light beam by an acoustic wave and to anacoustooptic modulation system comprising such an acoustoopticmodulation element.

A conventional acoustooptic modulation element of the type describedcomprises an acoustooptic medium for acoustooptically modulating asingle incident light beam by an acoustic wave. More particularly, theacoustooptic medium has a first surface for the incident light beam, asecond surface parallel to the first surface, and a third surfacebetween the first and the second surfaces. A transducer is attached tothe third surface so as to transduce an electric signal into theacoustic wave. When the electric signal has a plurality of frequencycomponents different from one another, the incident light beam which isadmitted through the first surface is modulated into a plurality ofoutput light beams by the acoustic wave resulting from the electricsignal. The output light beams are emitted through the second surface indirections dependent on the frequency components of the electric signal,as well known in the art. Each of the output light beams is recorded orprinted on a recording medium in a known manner.

With the acoustooptic modulation element, the number of the output lightbeams is determined by the number of the frequency components. Thismeans that each output light beam becomes weakened in intensity with anincrease of the frequency components. Accordingly, the intensity of eachoutput light beam fluctuates with the number of the frequency componentsincluded in the electric signal. Such fluctuation of each output lightbeam brings about unevenness of printing and inevitably deteriorates thequality of printing.

Another conventional acoustooptic modulation system comprises a beamsplitter in combination with a plurality of acoustooptic modulationelements. With this system, the beam splitter splits the incident lightbeam into a plurality of split light beams so as to supply theacoustooptic modulation elements with the split light beams,respectively. Accordingly, each of the acoustooptic modulation elementsindividually modulates each split beam by an acoustic wave resultingfrom an electric signal. As a result, a single modulated light beam isemitted from each acoustic modulation element in a direction dependenton each electric signal.

In order to produce a plurality of modulated light beams by the use ofthe above-mentioned system, a plurality of acoutooptic modulationelements should be arranged in parallel. The system therefore becomesbulky in structure. In addition, the acoustooptic modulation elementsshould individually be adjusted to the respective split light beamsemitted from the beam splitter. Such individual adjustment is verytroublesome.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an acoustooptic modulationelement which is capable of modulating a plurality of input light beamswithout any fluctuation of intensities.

It is another object of this invention to provide an acoustoopticmodulation element of the type described, which is compact in structure.

It is a further object of this invention to provide an acoustoopticmodulation system which can readily be adjusted to each of split beamsemitted from a beam splitter.

An acoustooptic modulation element according to this invention iscapable of responding to a plurality of input light beams substantiallyparallel to one another and electric signals equal in number to saidinput light beams for producing output light beams, respectively. Theacoustooptic modulation element comprises a single acoustooptic mediumblock having a first surface for the input light beams, a second surfacefor the output light beams, a third surface contiguous to the first andthe second surfaces, and a fourth surface contiguous to the first andthe second surfaces and opposite to the third surface and transducingmeans attached to the third surface and responsive to the electricsignals for transducing the electric signals into acoustic waves toproduce through the second surface the output light beams modulated bythe acoustic waves when the electric signals are given to thetransducing means.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 schematically shows a perspective view of an acoustoopticmodulation element according to a first embodiment of this invention;

FIG. 2 shows an enlarged and fragmentary front view of the acoustoopticmodulation element illustrated in FIG. 1;

FIG. 3 schematically shows a perspective view of an acoustoopticmodulation element according to a second embodiment of this invention;

FIG. 4 partially shows a perspective view of another acoustoopticmodulation element according to a modification;

FIG. 5 diagrammatically shows a view of an acoustooptic modulationsystem according to an embodiment of this invention;

FIG. 6 shows a top view of a beam splitter for use in the acousticmodulation system illustrated in FIG. 5;

FIG. 7 diagrammatically shows a perspective view of a system accordingto another modification; and

FIG. 8 diagrammatically shows a view of another acoustooptic modulationsystem according to a further modification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an acoustooptic modulation element 20 according toa first embodiment of this invention is supplied with first through n-thinput light beams 21₁, 21₂, . . . , 21_(n) substantially parallel to oneanother with a distance L between adjacent indicent light beams. Theacoustooptic modulation element 20 comprises a single block 22 of anacoustooptic medium, such as single crystal of tellurium dioxide, asingle crystal of lead molybdate, optical glass, or the like. The block22 is a parallelpiped of 20 mm×20 mm×10 mm, by way of example, and has afirst surface directed leftwards in FIG. 1, a second surface directedrightwards therein, and third and fourth surfaces directed upwards anddownwards, respectively. Each of the third and the fourth surfaces areopposite to the other and contiguous to the first and the secondsurfaces.

Each of the input light beams 21₁ to 21_(n) is incident on the firstsurface in parallel along an optical axis thereof.

A transducer member is attached to the third surface and is suppliedwith first through n-th electric signals which are in one-to-onecorrespondence to the input light beams 21₁ through 21_(n). Whenactivated by the electric signals, the transducer member producesacoustic waves, namely, ultrasonic waves, respectively. The transducermember is divided into first through n-th transducer units 23₁ to 23_(n)individually operable in response to the first through the n-th electricsignals. The first through the n-th transducer units 23₁ to 23_(n)extend in parallel along the respective input light beams 21₁ to 21_(n)on the third surface to individually propagate the acoustic waves intothe block 22.

Referring to FIG. 2 (which is a front view of the first surface inFIG. 1) together with FIG. 1, each of the first through the n-thtransducer units 23₁ to 23_(n) comprises a first electrode 26, atransducer medium 27, and a second electrode 28 which are successivelystacked on the third surface. The transducer medium 27 may be common toall of the first to the n-th transducer units 23₁ to 23_(n), assuggested by a broken line in FIG. 2, and may be, for example, a 36° Ycut plate made of a single crystal of lithium niobate. The firstelectrodes 26 each has a first electrode width in a direction transverseto each input light beam and is spaced from an adjacent first electrode.The second electrode 28 may have a second electrode width W somewhatless than that of the first electrode 26, as shown in FIG. 2. A pair offirst and the second electrodes 26 and 28 are electrically isolated fromthe remaining electrode pairs and are individually supplied with each ofthe electric signals.

Each of the input light beams 21 (suffixes omitted) is incident at anincident position on the first surface and passes through the block 22.The input light beams 21 are incident relative to wavefronts of theacoustic waves at Bragg angles which, as known in the art, aredetermined by dividing the product of wavelengths of the input lightbeams and frequencies of the electric signals by the velocity of sound.As shown in FIG. 2, the incident position is present on a center line ofthe second electrode 28 and has a depth D from the third surface. Thedepth D is, for example, 1 mm. Assume that each of the input light beamshas a spot size d.

When no electric signal is supplied across the first and the secondelectrodes 26 and 28, each of the first through the n-th input lightbeams 21₁ to 21_(n) is not subjected to any modulation and is sent fromthe second surface along the optical axis of each input light beam eachin the form of zeroth-order light beams 30₁, 30₂, . . . , 30_(n) shownby broken lines in FIG. 1.

Each of the acoustic or ultrasonic waves is propagated from eachtransducer unit 23 to the block 22 towards the fourth surface by supplyof each of the electric signals which are previously subjected toamplitude modulation in a known manner. As a result, each input lightbeam 21 is subjected to modulation by each of the acoustic waves toproduce first through n-th output light beams. Each of the first throughthe n-th output light beams includes a first-order diffracted light beam(shown by each solid line 31₁, 31₂, . . . , 31_(n) in FIG. 1) inaddition to the zeroth-order light beam 30. Such a first-order lightbeam 31 is sent to a printer (not shown) to be recorded on a recordingmedium.

With this structure, the acoustic waves are internally reflected fromthe fourth surface. Such reflection adversely affects modulation of eachinput light beam. In order to avoid the reflection from the fourthsurface, an acoustic absorber 33 is attached to the fourth surface. Theabsorber 33 may be of lead or the like.

Thus, a plurality of parallel light beams substantially parallel to oneanother can be modulated by the use of a single acoustooptic modulationelement.

Further referring to FIG. 2, consideration is made regarding thatdirectivity of each acoustic wave which is specified by velocitypotential, whose gradient to equal the velocity of the acoustic wave, asknown in the art. It is generally pointed out that the velocitypotential of the acoustic wave does not diverge within the acoustoopticmedium block 22, as shown by broken lines 36 in FIG. 2, on conditionthat the block 22 and the transducer unit 23 are surrounded by a rigidbody.

According to the inventors' experimental studies, it has, however, beenfound that the velocity potential of the acoustic wave objectionablydiverges within the acoustooptic medium block 22, as shown by solidlines 37 in FIG. 2, if the above-mentioned condition is not satisfied.

It is assumed in the illustrated acoustooptic modulation element thatthe distance L is less than five times the width W of the secondelectrode 28 and that one of the two adjacent light beams is modulatedby an acoustic wave with the other unmodulated by any acoustic wave. Forbrevity of description, a combination of the input and the output lightbeams may be simply called a light beam, as long as any confusion doesnot arise between them. Assume two adjacent light beams are selectedfrom the first through the n-th light beams and are designated as m-thand (m+1)-th light beams, respectively. Under the circumstances, it hasbeen observed that the (m+1)-th light beam is accompanied by a spuriousfirst-order diffracted component resulting from modulation of the m-thlight beam and that an on-off ratio, is reduced by the spuriousfirst-order diffracted component.

Such a spurious component results from the divergence of the velocitypotential of each acoustic wave. Therefore, the distance L must begreater than five times the width W of the second electrode 28 in orderto completely suppress any adverse influence on the adjacent lightbeams. As a result, the illustrated element 20 inevitably becomes bulkyin structure.

Referring to FIG. 3, an acoustooptic modulation element 20 according toa second embodiment of this invention comprises similar parts designatedby like reference numerals. As in FIG. 1, an acoustooptic medium block22' has a first or a front surface at the front in this figure, a secondor a back surface substantially parallel to the first surface andopposite to the first surface, and third and fourth surfaces directedupwards and downwards, respectively.

As shown in FIG. 3, the block 22' has first through n-th plateaus 41₁ to41_(n) separated from one another by channels 42 extending in parallelbetween the first and the second surfaces. Each plateau 41 has a topsurface providing a part of the third surface. In other words, thetotality of the top surfaces defines the third surface. Each of thechannels 42 is defined by a pair of internal side surfaces contiguous tothe first through the third surfaces and a recessed surface recessedfrom the third surface and contiguous to the internal side surfaces.

The input light beams are incident on the first surface of therespective plateaus 41₁ to 41_(n) while the output light beams appear onthe second surface of the respective plateaus. Each channel has a depthequal to or greater than the depth D of the incident positionillustrated in FIG. 2 and a width of, for example, 0.5 mm. The width ofthe channel is determined by the thickness of a wire saw used to formthe channels 42.

The first through the n-th transducer units 23₁ to 23_(n) are attachedto the first through the n-th plateaus 41₁ to 41_(n) on the top surfacesthereof, respectively. Each of the first through the n-th transducers23₁ to 23_(n) comprises the first electrode 26, the transducer mediums27, and the second electrode 28 which are successively stacked on thetop surfaces, as in FIG. 2. Also in FIG. 2, assume each second electrode28 has a second electrode width represented by W and is somewhatnarrower than the first electrode 26. The second electrode width W, thebeam distance L (shown in FIG. 1), and the depth D of the incidentposition (shown in FIG. 2) are, for example, 0.7 mm, 1.5 mm, and 1 mm,respectively.

Further referring to FIG. 3, in its block 22' has the fourth surface anarcuately recessed portion 45 recessed towards the third surface. Thearcuately recessed portion 45 extends between the first and the secondsurfaces. The arcuately recessed portion is nearest to the third surfacein a center region of each of the first and the second surfaces (shownin dotted lines) and gradually extends away from the third surface inend regions on both sides of the center region.

Such an arcuately recessed portion serves to reflect each acoustic wavein various directions. Therefore, each acoustic wave is notsubstantially returned back to each of the transducer units. It istherefore possible to avoid interference between the acoustic waves. Anacoustic absorber as illustrated in FIG. 1 may be attached to thearcuately recessed portion 45 and may be placed on the internal sidesurfaces and the recessed surfaces.

Referring to FIG. 4, the illustrated block 22" has a chevron shapedportion, namely, a triangularly recessed portion 46 on the fourthsurface. The triangularly recessed portion 46 is recessed towards thethird surface and is nearest to the third surface in the center region.It is possible with this structure to avoid interference between theacoustic waves.

Referring to FIG. 5, an acoustooptic modulation system according toanother embodiment of this invention comprises the acoustoopticmodulation element 20 illustrated in FIG. 1. The system comprises a beamsplitter 50 responsive to an incident light beam 51 for supplying theacoustooptic modulation element 20 with first through n-th split beamsas the input light beams 21₁, 21₂, . . . , 21_(n) illustrated withreference to FIG. 1, respectively. The split beams are emitted from thebeam splitter 50 in parallel. Let the incident light beam 51 be producedfrom an argon laser as an optical source 52 and have a wavelength of 488nm.

Referring to FIG. 6 together with FIG. 5, the beam splitter 50 comprisesa body 53 of quartz transparent to the incident light beam 51 and havinga thickness T and front and back surfaces 54 and 55 substantiallyparallel to each other. The incident light beam 51 is incident on thefront surface 54 at an incident portion. The front surface is coatedwith a reflection layer 56 except for the incident portion. The incidentportion is covered with an anti-reflection layer 57 with a part of theanti-reflection layer 57 superposed on the reflection layer 56.

The back surface 55 is divided transversely of the incident light beam51 into a plurality of areas which are equal in number to n and whichare therefore called first through n-th areas, respectively. Each of thefirst through the n-th areas is partially coated with each of firstthrough n-th layers 58₁, 58₂, . . . , 58_(n) having reflectivitiesgradually reduced from the first layer 58₁ to the n-th layer 58_(n).Such a reduction of the reflectivities is for emitting the first throughthe n-th split beams substantially equal to intensity to one another, aswill later become clear, and for partially reflecting light beamsinternally incident on the first through the (n-1)-th layers. For thispurpose, the first through the (n-1)-th layers 58₁ to 58_(n-1) may besemi-transparent or translucent and the n-th layer 58_(n) may betransparent. The first through the n-th areas may be completely orwholly covered with the first through the n-th layers 58₁ to 58_(n),respectively.

As shown in FIG. 6, the incident light beam 51 is applied to theincident portion on the front surface at a predetermined incident angleθ₀ and is transmitted towards the back surface. In this event, theincident light beam 51 is refracted at an angle θ₁ of refraction. Aninternally incident light beam on the first layer 58₁ is partiallytransmitted through the first layer 58₁ and partially internallyreflected towards the reflection layer 56. The reflection layer 56internally reflects a light beam internally incident thereon towards thesecond layer 58₂. Similar operation is repeated between the secondthrough the (n-1)-th layers 58₂ to 58_(n-1) and the reflection layer 56.Finally, the n-th split beam is emitted from the n-th layer 58_(n).

It should be recalled that the acoustooptic modulation element 20illustrated in FIG. 5 is supplied with the input light beams 21₁ to21_(n) with the distance left between two adjacent ones of the inputlight beams 21₁ to 21_(n). Accordingly, it is preferable that thedistance L₁ of the first through the n-th split beams is equal to thedistance L between two adjacent ones of the input light beams.

Inasmuch as the distance L₁ between two adjacent split beams iscalculated by:

    L.sub.1 =2T sin θ.sub.1                              (1)

and sin θ₁ is given in accordance with Snell's law by:

    sin θ.sub.1 =sin θ.sub.0 /n.sub.0              (2)

where n₀ is representative of the refractive index of the body 50.

Substitution of Equation (2) into Equation (1) gives:

    L.sub.1 =(2T/n.sub.0) sin θ.sub.0                    (3)

Thus, the distance L₁ between two adjacent split beams can be determinedby selecting the thickness T, the refractive index n₀, and the incidentangle θ₀.

Taking the above into consideration, the reflection layer 56, theanti-reflection layer 57 and the first through the n-th layers 58₁ to58_(n) are properly arranged on the first and the second surfaces.

Each of the reflection layer 56, the anti-reflection layer 56, and thefirst through the n-th layers 58₁ to 58_(n) can be formed by stacking aplurality of dielectric films. More particularly, the reflection layer56 has a thickness different from those of the anti-reflection layer 57and the first through the n-th layers 58₁ to 58_(n). The anti-reflectionlayer 57 also has a thickness different from those of the first throughthe n-th layers 58₁ to 58_(n). The reflection layer 56, theanti-reflection layer 57, and the first through the n-th layers 58₁ to58_(n) can be obtained by individually changing the number of thedielectric films, as known in the art.

As mentioned before, the reflectivities of the first through the n-thlayers 58₁ to 58_(n) are selected so that each of the first through then-th split beams becomes substantially equal in intensity to theremaining split beams. Let the reflectivities of the first through then-th layers 58₁ to 58_(n) be R₁, R₂, . . . , R_(n), respectively. It isassumed that the reflectivity of the reflection layer 57 and internalabsorption of the body 50 are equal to 1 and 0, respectively. In orderto render the intensities of the split beams equal to one another, thereflectivities R₁ to R_(n) should have relationships given by:

    (1-R.sub.1)=(1-R.sub.2)R.sub.1 =(1-R.sub.3)R.sub.1 R.sub.2 . . . =(1-R.sub.n)R.sub.1 R.sub.2 . . . R.sub.n-1               (4)

It is readily possible to form, by known technique, the first throughthe n-th layers 58₁ to 58_(n) so that Equation (4) holds.

Thus, the illustrated beam splitter can emit from the back surface,split beams substantially equal in intensity to one another. Inaddition, the split beams are substantially parallel to one another andsent from the beam splitter as the first through the n-th input lightbeams 21₁ to 21_(n) to the acoustooptic modulation element 20.

Referring back to FIG. 5, the acoustooptic modulation element 20comprises the first through the n-th transducer units 23₁ to 23_(n)attached to the third surface of the acoustooptic medium block 22, as isthe case with FIG. 1. The first through the n-th transducer units 23₁ to23_(n) are activated by first through n-th electric signals suppliedfrom an electric circuit 60, respectively.

The electric circuit 60 comprises an oscillator 62 generating anoscillation signal of 80 MHz. The oscillation signal is delivered tofirst through n-th modulators 64₁ to 64_(n) supplied with first throughn-th modulation signals S₁ to S_(n) from external signal sources (notshown), respectively. The first through the n-th modulators 64₁ to64_(n) carry out amplitude modulation to produce first through n-thamplitude modulated signals, respectively. The first through the n-thamplitude modulated signals are supplied through amplifiers 66₁ to66_(n) as the first through the n-th electric signals to the firstthrough the n-th transducer units 23₁ to 23_(n), respectively.

The acoustooptic modulation element 20 individually modulates the firstthrough the n-th input light beams 21₁ to 21_(n) by the acoustic wavespropagated from the first through the n-th transducer units 23₁ to23_(n) in the manner described in conjunction with FIGS. 1 and 2. As aresult, the first through the n-th output light beams depicted at 68₁ to68_(n) in FIG. 5 are emitted from the acoustooptic modulation element20.

The acoustooptic modulation element 20 illustrated in FIG. 3 may besubstituted for the element 20 illustrated in FIG. 5.

Referring to FIG. 7, the acoustooptic modulation element 20 is for usein combination with first and second optical systems 71 and 72 oppositeto the first and the second surfaces of the element 20 with spacingsleft between the first optical system 71 and the first surface andbetween the second optical system 72 and the second surface,respectively. Each of the first and the second optical systems may be acylindrical lens, an elliptic lens, or a spherical lens. The firstthrough the n-th split beams emitted from the beam splitter 50 aresharpened or reduced in spot sizes through the first optical system 71to be given as the first through the n-th input light beams 21₁ to21_(n). Likewise, each of the first through the n-th output light beams68₁ to 68_(n) is sharpened by the second optical system 72.

In general, an acoustooptic modulation element has a modulationbandwidth inversely proportional to spot sizes of each input light beam.

Accordingly, the first and the second optical systems 71 and 72 serve towiden the modulation bandwidth of the acoustooptic modulation element 20by reducing the spot sizes of the input light beams 21₁ to 21_(n).

Referring to FIG. 8, an acoustooptic modulation system according to amodification of this invention is similar to that illustrated in FIG. 5except that a combination of a grating 75 and an elliptic lens 76 issubstituted for the beam splitter 50 illustrated in FIG. 5. The grating75 is made of optical glass and has slits of 12,000/inch and a latticeconstant of 21,000 angstroms. As well known in the art, the incidentlight beam 51 is diffracted in directions determined by the grating 75.As a result, a plurality of diffracted light beams which are equal innumber to n are rendered parallel to one another through the ellipticlens 76 and are thereafter sent as the first through the n-th inputlight beams 21₁ to 21_(n) to the acoustooptic modulation element 20.Each of the first through the n-th input light beams 21₁ to 21_(n) issubjected in the acoustooptic modulation element 20 to modulation incooperation with the electric circuit 60 in the manner described inconjunction with FIG. 5. The combination of the grating 75 and theelliptic lens 76 may be called a beam splitter member. The grating 75may be of a transmission type or a reflection type.

A fiber grating may be substituted for the grating 75. Such a fibergrating is known in the art and comprises a plurality of optical fibersarranged in parallel with slits left between two adjacent ones of theoptical fibers.

While this invention has thus far been described in conjunction with afew embodiments thereof, it will readily be possible for those skilledin the art to put this invention into practice in various manners. Forexample, the first and the second optical systems 71 and 72 illustratedin FIG. 7 may be used in the system shown in FIG. 8.

What is claimed is:
 1. An acoustooptic modulation system responsive to asingle incident beam and a plurality of electric signals for producingoutput light beams, said acoustooptic modulation system comprising:agrating member for diffracting said incident light beam into a pluralityof diffracted light beams; and a single acoustooptic modulation elementresponsive to said plurality of the diffracted light beams as inputlight beams for acoustooptically modulating said input light beams inresponse to said electric signals to produce said output beamsdetermined by said electric signals.
 2. An acoustooptic modulationsystem responsive to a single incident beam and a plurality of electricsignals for producing output light beams, said acoustooptic modulationsystem comprising:beam splitting means for splitting said incident lightbeam into a plurality of light beams substantially parallel to oneanother; a single acoustooptic medium block having a first surface, asecond surface opposite said first surface, and a plurality of plateauswhich are equal in number to said input light beams and which arearranged in parallel between said first and said second surfaces so thata channel is left between two adjacent plateaus, each of said inputlight beams being incident on one of said plateaus at said first surfacewhile said output light beams exit from said plateaus at said secondsurface; and a plurality of transducing means each of which is mountedon one of said plateaus and each of which is responsive to one of saidelectric signals, for individually transducing each said electric signalinto an acoustic wave which is propogated into one said plateau to causethe output light beams at said second surface to be modulated by saidacoustic waves when said electric signals are applied to the respectivetransducing means.
 3. An acoustooptic modulation system as claimed inclaim 2 wherein said acoustooptic medium block has a recessed surfaceopposite said plateaus and contiguous with said first and said secondsurfaces, said recessed surface being recessed towards said plateaus. 4.An acoustooptic modulation system as claimed in claim 3, said blockbeing divided into a center portion and side portions on both sides ofsaid center portion in a direction parallel to the travel direction ofsaid input light beams, the distance between said third surface and saidrecessed surface being less at said center portion than at said sideportions.
 5. An acoustooptic modulation system as claimed in claim 3,wherein said recessed surface is of triangular shape.
 6. An acoustoopticmodulation system as claimed in claim 2, further comprising acousticallyabsorbing means attached to the surface opposite said plateaus forabsorbing said acoustic waves.